This invention relates to improved radar apparatus and, more particularly, to a rotatable radar structure that is of compact design and is formed with a minimum number of microwave and electrical components.
Conventional radar apparatus now available for marine applications, and particularly adapted for the so-called recreational market (e.g. non-commercial and non-military uses) is provided with two parabolic antennas, one for transmitting radar signals and the other for intercepting reflected radar signals. In this design, the antennas are disposed one atop the other; and for proper operation, the two antennas should be of identical configuration and should exhibit exact alignment with respect to each other. One disadvantage associated with such dual antennas arises out of the fact that if there is significant mechanical tolerance in the antenna assemblies, skewing between the transmit and receive structures may result, thus reducing the accuracy of detecting and displaying the reflected radar signals.
Another disadvantage associated with the aforementioned dual antenna structure arises out of mechanical vibration of the radar unit. Such vibration may result in deterioration of the mechanical components causing serious faults and possibly mechanical failure. To minimize such vibration, it is desirable to locate the mass of the radar structure at or close to the axis of rotation. This, however, is not easily attained in the dual parabolic antenna arrangement.
Yet another disadvantage associated with parabolic antennas relates to the side lobes of the transmit/receive pattern. These side lobes constitute a significant factor in false target appearances.
The aforementioned disadvantages associated with parabolic transmit and receive antennas generally have been overcome with the introduction of the slotted wave guide antenna. In particular, a single slotted wave guide antenna may be used both for transmission and reception of radar signals, thereby obviating the need for precise assembly and mounting of two separate antenna structures. Also, very low side lobes are present in the slotted wave guide antenna, resulting in a marked improvement. As a result, signal loss is reduced and the need for significantly higher power, as required previously by dual parabolic antennas, has been reduced. Still further, the presence of crosspolar interference, which is due to background reflections and which is most significant at a 45.degree. angle in parabolic antennas, is substantially reduced when using a slotted wave guide antenna. This crosspolar interference is further reduced when a slotted filter is positioned in front of the slotted wave guide.
One proposed radar assembly which incorporates the aforementioned slotted wave guide antenna is comprised of a transmitter, such as a pulse modulator and a magnetron, a circulator which couples the radar pulses generated by the magnetron to the antenna and which also couples reflected radar signals from the antenna to a receiver, and a limiter which is disposed between the circulator and the receiver. In this type of arrangement, tunable wave guides connect the magnetron to the circulator, the circulator to the limiter and the limiter to the receiver. Typically, the magnetron, circulator, limiter and receiver all are fixedly supported in a suitable cabinet, or housing, leaving only the slotted wave guide antenna to rotate. A conventional rotary drive is used to rotate the antenna; and a so-called rotary joint provides a microwave coupling between the antenna and the circulator. However, it has been found that the rotary joint which is needed to permit the antenna to rotate while providing microwave coupling between it and the circulator, is a source of power loss. Furthermore, a tunable wave guide must be used to connect the circulator to the rotary joint and that joint itself must be tunable. In the absence of precise tuning of each of the aforementioned wave guides, signal loss may be present and false echo signals may be displayed.
Typically, simple electrical wires are used to connect signal processing circuitry to the transmit modulator for supplying proper control and trigger pulses to that modulator, and also to connect the signal processing circuitry to the receiver in order to process and display the reflected radar signals. Heretofore, a minimum of six wires had been used for such an electrical interconnection, one wire to supply operating voltage to the modulator, another wire to supply ground or reference potential to the modulator, another wire to supply trigger signals to the modulator, yet another wire to supply operating potential to the receiver, a further wire to supply ground potential to the receiver and an additional wire to receive the information (or video) signals from the receiver. In addition, a DC pulse width control signal normally is supplied over yet a further wire to the modulator and another wire is used to supply a gain and/or sensitivity control signal to the receiver. As a result, six or eight wire interfaces have been provided between the radar transmit/receive circuitry and the signal processing circuitry. This has added to the difficulty and expense in constructing and assembling radar devices.
Recently, a unitary device has been introduced which combines, in one structure, the magnetron, circulator and limiter which previously had been constructed as individual devices. This unitary magnetron, circulator and limiter device is manufactured by the English Electric Valve Company, Limited and has been identified with the trademark "DUPLETRON". Although the use of a "DUPLETRON" device offers the advantage of reducing construction and assembly costs and reduces the size and space requirements of the microwave components included in a typical radar assembly, the aforementioned difficulty associated with the rotary joint nevertheless is present.